It is one of objects of a next generation wireless communication system to provide various data services as well as voice service. In order to provide the various data services, it is necessary to develop a high speed data communication technology that supports a data transmit rate of several Giga-bits per second (Gbps).
A Multiple Input Multiple Output (MIMO) system uses a plurality of transmitting and receiving antennas for transmitting and receiving data. The MIMO system may incredibly increase channel capacity compared to a system using one transmitting antenna and one receiving antenna.
In order to realize such theoretical channel capacity gain of the MIMO system, various methods were introduced, such as Spatial Multiplexing and Space-Time codes.
The Spatial Multiplexing is a technology for simultaneously transmitting a plurality of data through different transmitting antennas. Therefore, the Spatial Multiplexing increases system capacity without system bandwidth increased additionally. Vertical-Bell Laboratory Layered Space-Time is one of the representative schemes thereof.
The Time-Space Code is a technology for obtaining diversity and coding gain at the same time by assigning proper codes to a data stream and transmitting data at a plurality of time slots through a plurality of antennas. Space-Time block codes is one of representative schemes thereof.
For example, in a 2×2 MIMO system, a receiver receives a signal transmitted through one or more antennas from a transmitter by properly dividing the received signal. Therefore, the receiver can receive two data streams at one time slot. Also, a data error rate can be reduced by transmitting two data streams through two antennas.
That is, the MIMO system having multiple transmitting and receiving antennas can transmit at least one of data streams at the same time because of increment of transmission paths. Also, a receiving error is not generated from all of transmission paths or one predetermined wireless path.
Auto Repeat reQuest (ARQ) is a method for retransmitting corresponding data at a transmitting end when error is generated from received data at a receiving end.
Although ARQ has comparative simple structure, the ARQ provides high reliability. For example, if a receiver detects error from received data using a cyclic redundancy check (CRC) code which has superior error detection capability, a transmitter requests retransmission of the data. As a result, the reliability is improved through re-transmission.
ARQ is classified into Stop and Wait (SW), Go-Back-N (GBN), and Selective Repeat (SR). The SR advantageous has high data transmit efficiency compared to the SW and the GBN. However, the realization complexity of the SR is very high.
In ARQ, data retransmission is generally performed at a medium access control (MAC) layer. The MAC layer can retransmit a corresponding MAC Protocol Data Unit (MPDU) as many times as a predefined maximum retransmission time. If it fails to re-transmit the data unit within the maximum retransmission time, the corresponding data unit is discarded.
Therefore, whether to successfully retransmit a data unit within the maximum re-transmission time is a very important factor because system performance depends thereon.